1. Field of the Invention
This invention relates to improvements in the field of test devices or elements for evaluation of a fluid sample, such as a biological liquid. In particular, this invention relates to devices or elements incorporated with assay reagents, including those for homogeneous specific binding assays.
2. Brief Description of the Prior Art
Test strips and similar solid state analytical elements have become commonplace in the analysis of various types of fluid samples, particularly biological liquids. They have been advantageous in, for example, the diagnosis of disease. They have been known and used for many years in a wide variety of fields, particularly as in vitro diagnostic devices for the detection of various urine and blood components such as glucose, protein, occult blood and so forth. For example, see U.S. Pat. Nos. 3,012,976; 3,164,534; and 3,485,587.
Test devices have been prepared which can be used to perform specific binding assays. Specific binding assays are useful for determining various organic substances of diagnostic, medical, environmental and industrial importance which appear in liquid mediums at very low concentrations. They are based on the specific interaction between the bindable analyte under determination, also referred to as a ligand, and a binding partner therefor. Where one of the analyte and its binding partner is a hapten or antigen and the other is a corresponding antibody, the assay is known as an immunoassay. For example, see U.S. Ser. Nos. 255,521 (now U.S. Pat. No. 4,447,527), 202,378 (filed Oct. 30, 1980, and now abandoned), and 253,147 (now U.S. Pat. No. 4,442,204), each of which discloses such a specific binding assay device and is commonly assigned herewith.
These test strips and similar devices have been compromised in the reliability of the results they provide when the application of sample has caused the migration of reagents away from the point of sample application to the device. This causes a nonuniformity of reagent concentrations throughout the device and resulting nonuniformity of detectable signal or response level. Because of this, a single device, which has been contacted with a single sample, can present an infinite variety of signal levels depending on which portion of the device surface is used to take a reading.
The approach which has hetertofore been used in attempts to avoid what has been referred to as a "ringing" effect has been the use of spreading layers. For example, see U.S. Pat. No. 3,992,158 and U.K. Patent Application GB No. 2,052,057. They assert that because spreading occurs in the spreading layer and the spread sample is presented to the reagent-containing layer without lateral hydrostatic pressure, the "ringing" phenomenon is avoided. These layers add complexity and expense to the manufacture of test devices.